Implantable medical devices, such as implantable cardiac rhythm management devices (e.g., pacemakers, defibrillators, and cardioverters), monitor and provide therapy to the heart of a patient that suffers from a cardiac arrhythmia. For example, in an attempt to maintain regular rhythm, an implantable device may track the type and timing of native cardiac signals generated by the heart. In this way the implantable device may determine whether cardiac events (e.g., contractions) are occurring and whether they are occurring at the proper times. In the event contractions are not occurring or are occurring at undesirable times, the implantable device may stimulate the heart in an attempt to restore proper rhythm. For example, an implantable device may stimulate the cardiac muscles of one or more of the chambers of the heart by delivering electrical pulses via one or more leads implanted in or near the chamber(s).
An implantable device also may track cardiac signals via one or more leads that are implanted in or near one or more of the chambers of the heart. Here, through the use of amplification, threshold detection and filtering, signals received via the leads may be associated with a particular cardiac event. These cardiac events may include, for example, P-waves, R-waves, and T-waves. A P-wave corresponds to a contraction (depolarization) of an atrium. A QRS complex (comprising an R-wave) corresponds to a contraction (depolarization) of a ventricle. A T-wave corresponds to a return to a resting state (repolarization) of a ventricle.
In some implementations an implantable device may employ a sense amplifier and a threshold detector for cardiac event detection. In some aspects, the sense amplifier senses cardiac signals and provides the sensed signals to the threshold detector.
The sense amplifier may include or be associated with a signal filter. Here, the bandwidth of the filter may be selected to allow the signals that the system is attempting to detect to pass through the filter. In general, the filter is designed in a manner that tends to reject any other signals. That is, other signals that are not of interest may not pass through the filter or may be significantly attenuated by the filter.
The signals that pass through the filter may be provided to the threshold detector. The threshold detector generates an output signal in the event the amplitude of the input signal exceeds a fixed threshold level or a threshold level defined by an automatic sensitivity control scheme. The output signal may thus be taken as an indication that a certain cardiac event has occurred. The output by the threshold detector may thus indicate detection of a P-wave, an R-wave, or some other signal.
By analyzing the type and timing of these indications the implantable device may determine whether stimulation pulses need to be generated as noted above. Thus, if the implantable device detects cardiac events at the appropriate relative times, the device may simply continue monitoring the received indications. In contrast, if an indication has not been received for a predefined period of time, the implantable device may deliver an appropriate stimulation (e.g., pacing) pulse to the heart. Alternatively, in the event a tachycardia condition is detected, the implantable device may deliver a stimulation shock to the heart.
In some aspects, the accuracy with which an implantable device identifies cardiac events may depend on appropriate programming of atrial and ventricular sensitivities as well as refractory and blanking periods. For example, for atrial sensing a post-ventricular atrial refractory period (“PVARP”) may be employed to avoid inappropriate sensing of retrograde conducted P-waves. In addition, a post-ventricular atrial blanking (“PVAB”) period may be employed to prevent over sensing of far-field QRS complexes and to improve detection of atrial fibrillation. Here, sensed atrial events that fall within the PVAB period may be ignored by, for example, a mode switch algorithm that monitors the peak-to-peak interval in an atrial refractory period (PVARP or AV-PV delay) to trigger a mode switch when atrial fibrillation is detected. Also, a pre-ventricular atrial blanking (“PREVAB”) period may be used to improve far-field R-wave discrimination. For ventricular sensing, a ventricular refractory period (“VREF”) may be employed to avoid double-counting of ventricular depolarization and repolarization (e.g., QRS complexes and T-waves, respectively).
Techniques such as those described above may not always provide a proper indication of cardiac events. For example, a P-wave detection circuit may indicate a detection based on a true P-wave in some instances while, in other instances, the circuit may improperly indicate a detection in response to reception of a far-field R-wave, a far-field T-wave, extracardiac physiologic noise, or external noise. Similarly, an R-wave detection circuit may indicate a true R-wave in some instances and, in other instances, improperly indicate a detection in response to reception of a T-wave, redetection of the same R-wave, or noise. Thus, in practice, the selection of appropriate sensitivity thresholds and blanking periods may involve a complex trade-off between P-wave sensing performance, atrial fibrillation sensing performance, and far-field oversensing.